Hydrological Sciences—Journal—des Sciences Hydrologiques, 43(4) August 1998
Special issue; Monitoring and Modelling of Soil Moisture: Integration over Time and Space
579
The mechanism of soil water movement as
inferred from 18 0 stable isotope studies
J. C. GEHRELS & J. E. M. PEETERS
Netherlands Institute of Applied Geoscience TNO, National Geological Survey, PO Box 6012,
2600 J A Delft, The Netherlands
e-mail: [email protected]
J. J. DE VRIES
Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
M. DEKKERS
Provincie Limhurg, Limburglaan 10, 6229 GA Maastricht, The Netherlands
Abstract The seasonal movement of soil water through a deep vadose zone of a
sandy hill was studied in the central part of The Netherlands. The behaviour of soil
water was tracked by monitoring soil water fluctuations and changes in the
environmental tracer lS 0 in vertical profiles at test sites with grassland, heathland
and forest. In the temperate climate of the study area a soil moisture deficit develops
during summer and a precipitation surplus prevails during winter. The 180 content in
precipitation exhibits a distinct seasonal cycle with the highest (relatively enriched)
values of c. -6%o vs V-SMOW occurring in summer and the lowest (relatively
depleted) values of c. -9%o vs V-SMOW in winter. The influence of this cycle can
be traced back in the subsurface to a depth of sometimes more than 6 m. This
seasonal fluctuation moves around the average annual 180 content in rainfall, tending
closer to the average as depth increases. In the saturated zone the same average
value is observed. This suggests that, although the precipitation surplus is limited to
the winter period, part of the summer precipitation also infiltrates to below the root
zone and contributes to the groundwater recharge. The results of this study indicate
that preferential flow dominates in the root zone. Below the root zone of low
homogeneous vegetation covers, the soil water movement is dominated by diffuse
flow. In forests, a more heterogeneous root pattern causes a more preferential type
of flow, also at greater depths below the root zone.
Mécanisme du mouvement des eaux souterraines
d'après les
conclusions des études de l'isotope stable lsO
Résumé Nous avons réalisé une étude du mouvement saisonnier des eaux
souterraines à travers une zone vadose profonde d'une colline sableuse, au centre des
Pays-Bas. Le comportement des eaux souterraines a été suivi en contrôlant les
modifications du traceur environnemental 180 selon des profils verticaux sur des sites
d'essai situées dans des prairies, des landes et des forêts. La teneur en 1S0 des
précipitations présente un cycle saisonnier propre, avec des valeurs maximales
(relativement enrichies) d'environ -6%o par rapport au V-SMOW l'été, et minimales
(relativement appauvries) de -9%o par rapport au V-SMOW l'hiver. L'influence de
ce cycle peut être retrouvée dans le sous-sol jusqu'à une profondeur parfois
supérieure à 6 m. Cette fluctuation saisonnière est proche du pourcentage annuel
moyen de 180 dans les précipitations, tendant à se rapprocher davantage de la
moyenne lorsque la profondeur augmente. Dans la zone saturée, on observe la même
valeur moyenne ce qui suggère qu'une partie des pluies estivales s'infiltre sous la
couche d'enracinement et contribue à la réalimentation des nappes phréatiques, bien
que l'excédent de précipitations se limite à la période hivernale. Il ressort des
résultats de cette étude que l'écoulement préférentiel est dominant dans la couche
Open for discussion until 1 February 1999
./. C. Gehrels et al.
580
d'enracinement. Sous la couche d'enracinement des couvertures végétales
homogènes basses, le mouvement des eaux souterraines est dominé par un
écoulement diffus. Dans les forêts, la disposition plus hétérogène des racines
entraîne un type d'écoulement préférentiel, y compris en dessous de la zone
d'enracinement.
INTRODUCTION
Transport through the unsaturated zone is a complex nonlinear and inhomogeneous
flow process and therefore difficult to model on the basis of Darcy's law exclusively.
Tracer studies are an important aid to follow and reconstruct flow behaviour, notably
if tracers are part of the water molecule like 18 0. The isotopic fluctuations in
precipitation for a given location are mainly controlled by climatic parameters,
especially temperature and provenance of the rain producing air mass (Dansgaard,
1964). As a result, the isotopic composition of rainfall in cool or temperate climates
is normally subject to a distinct seasonal pattern. Although the isotopic composition
of rainfall differs from event to event, winter precipitation is generally more depleted
in 180 and therefore lighter than summer precipitation.
Only very few studies with the stable isotopes 180 and 2H have been carried out
under temperate climatic conditions. In arid climates, a clear cyclicity in the soil
water isotope content is usually not observed because the low recharge rates will
ensure that the peaks and troughs in the isotope concentrations will be separated by
very short distances in the soil profile. However, under temperate conditions, infiltration rates are much higher, so that seasonal fluctuations are more easily detected.
Thoma et al. (1979) were the first to observe that the seasonal variations of 2H in
rainfall is preserved in the unsaturated zone. Saxena (1987) successfully traced the
vertical shift of soil moisture from winter precipitation in an area where a distinct
seasonal variation in 180 was observed between the lighter (depleted in 180) winter
snowmelt water and heavier summer rainfall.
The aim of this study is to reveal the infiltration pattern using the seasonal
character of the 180 content in rainfall. This seasonal character is used to trace the
seasonal infiltration pattern, the flow velocity and the mechanism of soil water flow.
The 1S0 isotope is used as a tracer to follow the movement of soil water on its way
down to the groundwater table in a thick, unsaturated zone of an ice-pushed hill in
the central part of The Netherlands (The Veluwe area). The climate is semi-humid
with a uniform rainfall distribution throughout the year of c. 900 mm. A soil
moisture deficit develops during summer, whereas a precipitation surplus between
250 and 400 mm prevails during winter, depending on the vegetation. Three different
sites with similar subsoil but different types of vegetation were investigated: grass,
heather and forest.
The 8180 content of precipitation and soil water was compared with the ô180
content of groundwater. Since a precipitation surplus only exists during winter time
(a precipitation deficit develops during summer time), one would expect the
groundwater 180 composition to be inclined towards the lighter winter time
precipitation composition. It was found, however, that the average 180 content in soil
The mechanism of soil water movement as inferred from
ls
O stable isotope studies
5 81
water and groundwater are almost equal to the average 8180 level in precipitation.
The results of this study indicate that preferential flow bypassing the root zone is an
important mechanism for the infiltration of summer precipitation.
SIMULATING DIFFUSE INFILTRATION USING lsO
In this section a method is proposed to simulate diffuse infiltration from an S l8 0 soil
water profile. The method is based on identification of seasonal peaks and calculation
of the soil water flux through vertical displacement between successive seasonal
inputs of recharge water with their respective isotopic characters. The variations in
natural abundance of oxygen (and hydrogen) isotopes in water samples are usually
expressed as deviations from an international standard. The results are given here in
5 permil (%o) units relative to V-SMOW (Vienna Standard Mean Ocean Water) as:
ô , 8 0:
"sample
^ V S M O W / """VSMOW
1000
(1)
where R is the isotopic ratio of ,8 0/ 16 0. Variation in abundance will occur between
the different phases of water. When water changes physical state as it evaporates,
condenses or freezes, water molecules containing heavier atoms such as 180 and 2H
are preferentially concentrated in the less mobile phase (Dinçer & Davis, 1984).
Therefore both the stable isotopes 180 and 2H exhibit a clear seasonal cycle in
precipitation that can theoretically be followed through the unsaturated zone.
The principle of translating the precipitation isotope record to an isotope depth
profile is similar to the methods used in a number of tritium studies but, to our
knowledge, not previously carried out for 18 0. The method that is proposed here will
be called the 8180 time-depth model and proceeds as follows. The 180 profile is used
to estimate the average infiltration velocity vÏM, of the soil water through the profile,
namely, v,,„ is calculated from vv,„ = dit where the profile depth d is related to the
travel time t prior to the sampling date, using the correlation between the cyclicity in
the 180 profile and in the 180 precipitation record. The translation is completed by
including the amount of precipitation and soil water content. For each time step,
from the sampling date back in time (the first time step being the first depth
increment and so on), the associated depth interval is calculated according to:
d(0
_ (Pit)
e ï
where t is the time elapsed prior to the sampling date, P(t) the precipitation amount
of the rth month, P the average monthly precipitation, 0(d) the soil water content at
the calculated depth d and 6 the average soil water content of the profile. Thus
P(t)IP and Q/B(d) are dimensionless correction factors. The calculation includes an
iterative procedure, as 9(d) can only be obtained indirectly.
The 5180-time curve of precipitation is thus rescaled to the 8lsO-depth curve of
soil water by calculating the soil water velocity per time step from the precipitation
582
J. C. Gehrels et al.
amount and the soil water content. The obtained soil water velocity gives the depth to
which the soil water has moved in the corresponding time step. Subsequently, an
exponential damping is introduced to account for a dispersive attenuation of the
isotopic fluctuations:
d[lO(d) = 8]fO + (8 )fO(t) - §;?0) • Q-',d
(3)
where 8\swO(d) is the attenuated 8^,0 at depth d, with d calculated from
equation (2); 8pO(f) is the S',80 at I time steps prior to the sample date; 8'|0 is the
average 8180 content in precipitation and b the exponential attenuation coefficient. The
parameters vsw and b can be estimated with a least squares optimization algorithm and
the recharge is then again calculated from the obtained vsw and observed 9.
DISTURBANCE OF THE ISOTOPIC PATTERN IN THE UNSATURATED
ZONE
The isotopic composition of infiltrated precipitation is subject to changes while
transported from above the canopy to below the root zone, so that the relation with
the precipitation record may become obscured. The main processes that possibly
affect the soil water isotope content can be summarized as (a) isotopic fractionation,
(b) selective root uptake of precipitation, and (c) the mechanism of soil water flow.
These processes are discussed below.
Fractionation by interception evaporation and throughfall
Incomplete evaporation from a wet canopy, quickly followed by a new shower
washing off the highly enriched rainwater, may cause a relatively high enrichment
compared to a situation where the canopy is entirely dry before the next storm
arrives. Saxena (1986) carried out a throughfall study in a dense pine forest near
Uppsala (Sweden) and observed that throughfall was enriched with respect to rainfall
by about 0.3 %>. These results closely agree with the findings of DeWalle &
Swistock (1994), who measured enriched 8180 in throughfall and rainfall in
hardwood and coniferous forests in Pennsylvania (USA). Thus the influence on
average isotopic composition in the subsoil seems limited to c. 0.3 %> for coniferous
forests. For The Velu we area, largely covered with forest, the groundwater
composition is therefore expected to be slightly enriched with respect to
precipitation. Interception will not play a significant role for lower vegetation types.
Fractionation by soil evaporation
Direct evaporation from the soil causes enrichment in the 8180 fraction of the
remaining water (Zimmermann et al., 1967). The effects of isotopic fractionation by
The mechanism of soil water movement as inferredfrom lsO stable isotope studies
583
bare soil evaporation are in general relatively low for European temperate conditions, the effects of enrichment being highest for coarse gravel and lowest for fine
sands (Saxena, 1987). In humid areas soil evaporation is effectively reduced below
vegetated soils and isotopic fractionation is then negligible.
Fractionation by root water uptake
Zimmermann et al. (1967) studied the effect of transpiration with an experiment in
which the root water uptake of various plant species was monitored. No fractionation
was found in the remaining soil water after 50% of the water had been consumed.
Allison et al. (1984) tested a much greater fraction of root water uptake, up to
extreme suction conditions. Even under these conditions, rarely encountered under
temperate circumstances, no conclusive proof of fractionation by root water uptake
was found.
Selective root uptake of precipitation
Evapotranspiration has a marked cyclic pattern, being almost nil in winter and
highest in summer. Therefore it seems very likely that groundwater recharge consists
for a larger part of isotopically lighter winter precipitation than of summer
precipitation. However, according to Mook (1988) the observed values of ôlsO of
groundwater in The Netherlands and other areas with comparable moderate climates
are in reasonable agreement with those of the average annual rainfall. The explanation of the average groundwater stable isotope composition is related to the
mechanism of soil water flow, as discussed in this paper.
Dispersion, soil heterogeneity and preferential flow
Dispersion and preferential flow will affect the shape of the soil water isotopic
profile. In contrast with the factors described earlier, it is not the isotope content that
changes, but the water flow is affected. Many studies have shown that the
fluctuations in the 8 values are smoothed with depth. Normally, the models that
describe the soil water isotope profile are based on assumptions similar to the
majority of models for soil water movement. It is generally assumed that water
moves through the soil in layers, by way of piston displacement, with some mixing
between horizons due to dispersion. However, in reality soil water may flow via
preferential pathways.
Preferential flow may originate from a number of causes, such as soil
heterogeneity (Kung, 1990), cracks and macropores (Beven & Germann, 1982),
shrinking and swelling in clay and peat soils (Bronswijk, 1991) or unstable wetting
fronts and fingering in water repellent sandy soils (Ritsema et al., 1993). If piston
flow were entirely absent and all infiltration were dominated by preferential flow,
J. C. Gehrels et ai
584
seasonal cycles would not be discernible in the unsaturated zone. In contrast with the
cited literature on preferential flow, Dincer & Davis (1984) state that in coarse sandy
soils with high infiltration rates, piston flow models are generally valid, because
water moves downward as a wetting front with little longitudinal or transversal
dispersion. The results of the isotope profiles as presented here provide insight into
the question on piston displacement vs preferential flow and show that isotope
profiles can give useful information on the character of infiltration in general.
ô180 CONTENT OF PRECIPITATION, SOIL MOISTURE AND
GROUNDWATER
In The Netherlands, data on stable isotope content of precipitation are collected by
the Centre for Isotope Research (CIO) at Groningen. The composition of
groundwater is known from a number of previous studies. No data on the stable
isotope composition of soil water in The Netherlands are available from literature.
Isotopic composition of precipitation
8 ,8 0 precipitation data have been sampled at a limited number of stations in The
Netherlands. From Groningen a time series is available from 1964 onward. At other
stations closer to The Veluwe, data collection stopped in the 1980s or early 1990s:
De Bilt (1981-1991) and Deelen (1981-1986). Groningen is therefore the only
station that can be used in the present study that has measurements in 1993 and 1994.
Differences in ô180 values are likely to exist between Groningen and The Veluwe
(-120 km apart) and may vary greatly between separate rain storms. However,
monthly values are expected to have a similar pattern, even over distances of a few
Table 1 Average precipitation , 8 0 values at Groningen (measured from 1964 onwards), De Bilt
(1981-1991) and Deelen (1981-1986). For comparison, averages at Groningen were calculated for the
same periods as those for De Bilt and Deelen.
Groningen
De Bilt
Groningen
Deelen
Groningen
Groningen
Groningen
1964-1995
1981-1991
1981-1991
1981-1986
1981-1986
1992-1994
1993-1995
P
(mm month1)
Simple*
64.5
71.3
65.4
74.6
64.7
69.0
82.6
-7.60
-7.24
-7.40
-7.42
-7.19
-7.21
-7.23
18
s o
Weighted1
CT
â'«0
1.92
1.78
1.78
2.11
1.90
1.73
1.77
8180
-7.79
-7.80
-7.67
-7.83
-7.49
-7.32
-7.59
18
t unweighted average over the monthly 0 values; the standard deviation CF-,8 is almost entirely
determined by seasonal changes and can thus be seen as a measure for the seasonal amplitude.
t weighted to monthly precipitation amounts.
The mechanism of soil water movement as inferred from !S0 stable isotope studies
585
-10-10.5-"—i
Jan
1
1
1
1
<
1
1
1
1
1
'
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
1—
Jan
Fig. 1 Average long-term seasonal cycle of Groningen (1964-1995) data, shown
with standard error bars and fitted cosine function (equation (4)).
hundreds of kilometres (Mook, 1988; Rozanski et al, 1982). In Table 1 the average
isotope contents at the three stations are compared to illustrate the degree of spatial
variability. The averages at the three stations are highly comparable within tenths of
%o of S180. The long-term (1964-1995) unweighted average precipitation content at
Groningen is 18S = -7.6%o and the weighted average 18ô = -7.8%o (Table 1).
In Fig. 1 monthly average 8180 values are shown for Groningen, nicely
demonstrating the seasonal character. A cosine function was fitted to the monthly
data to emphasize and quantify the cyclicity. The unweighted least-squares fit for the
Groningen data over the period 1964-1994 resulted in:
8l8O = -7.61(±0.08)-1.56(±0.11)cos{27i[t-0.053(± 0.012)]}
(4)
The highest 5180 values are found during summer and the lowest during winter. A
fitted amplitude A of l.56%o indicates that the difference between summer and
winter is on average ~3%o. The standard errors in the monthly mean values range
from -0.2 %o in summer to -0.3 %o in winter.
Isotopic composition of groundwater
A number of sources are available for assessment of the ô180 isotopic composition of
groundwater at The Veluwe. Mook (1988) tabulated a number of groundwater
samples from The Veluwe area and observed values between -7.3%o and -8.3%o,
with an average of -7.8%o at sampling depths between 0 and 100 m below the
groundwater table. The groundwater ô180 content therefore compares remarkably
well with the long-term weighted average precipitation isotope content of
8180 = -7.8%o found for Groningen (Table 1). In the present study groundwater was
586
J. C. Gehrels et al.
sampled from the saturated zone in the near surroundings of the study sites. The
observed average composition was -7.53%o (<j^ls = 0.07%o) which is reasonably
ô O
close to the value of -7.8%o reported by Mook (1988). Van Kessel (1993) surveyed
the 5180 isotope composition at a small number of wells close to the present study
sites at various depths up to 400 m. The observed average value of -7.52%o
(CT_18 = 0.08%o) is very close to the present findings, although the present samples
5 O
were only taken at shallow depths (less than -100 m).
Sampling and analysis
The field samples were taken in the summer of 1993 and bimonthly throughout 1994.
Soil samples were collected in cores of 5 cm length at various depths up to 8 m. For
the subsequent extraction of the pore water from the soil samples a number of
laboratory techniques have been reported, such as vacuum distillation, azeotropic
distillation and centrifugation (Walker et al, 1994). In the present study soil water
was extracted by vacuum distillation and centrifugation. Vacuum distillation is
considered as a reliable and accepted method (Saxena, 1987; Gieske, 1992).
Centrifugal extraction (e.g. Bath et al., 1982; Wellings, 1982) is time-reducing and
cheap and for these reasons several samples in this study were extracted by
centrifugation. In 1993 a part of the samples was treated with both centrifugal
extraction and vacuum distillation. In 1994 only vacuum distillation was used. For a
more thorough description of the extraction procedures the reader is referred to
Gehrels (in preparation). Once the soil water was extracted, the samples were
analysed at the Centre for Isotope Research (Groningen) using a special isotope ratio
mass spectrometer according to the standard technique described by Mook & Grootes
(1973). The total accuracy of soil water extraction, preparation at the 18Opreparation
system and measurement with the mass spectrometer was estimated at -0.3 %0
(H. A. J. Meijer, CIO, personal communication).
RESULTS
The influence of vegetation on the ô180 soil water profile
The 8180 isotopic composition of soil water may be influenced by a number of
factors that were described earlier in this paper. These factors, summarized as
isotopic fractionation, selective root uptake and flow mechanism, are all related to
the vegetation. Tracers such as 180 therefore provide an insight into how the
vegetation influences the flow mechanism.
In Fig. 2 profiles are presented that were drilled below different vegetation types
in August (solid) and September (dashed) 1993. The average 8180 value for
groundwater at The Veluwe is indicated with the dotted lines that mark the value of
5„!,80 = -7.5%o; the groundwater table at the test sites is at c. 20 m below the
The mechanism of soil water movement as inferred from
-it
ls
O stable isotope studies
587
purple moor grass
July 1992
-4
».
-5'
-6
I V'
-7
1
r^\
August 1993
V A
VV*".
J^j'^l—m m
f*3>
S
-9
m
/
(a)RK
mixed forest
coniferous
—~
August
August
.«...
September
September
%M$I*\^~m
»
£
-V, J! S ^ f * - - %-*^>i
Ï (0 UB
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
depth (m)
depth (m)
ls
Fig. 2 O profiles drilled in August and September 1993 (and one in M y 1992)
below purple moor grass (Radio Kootwijk), heather (Asselsche Heide), deciduous
forest (Amersfoortse Weg), pine forest (Woeste Hoeve) and mixed forest
(Ugchelsche Bos). Dotted lines indicate average groundwater value of - 7 . 8 .
surface. The profiles below the low vegetation types of purple moor grassland and
heathland exhibit a visible seasonal pattern in 180 content up to a depth of 5-6 m. A
clear bulge of enriched summer infiltration is visible. The summer profiles of 1992
and 1993 at Radio Kootwijk show a marked resemblance to each other. The 180
profiles below the forested sites, however, lack a clear seasonal cyclicity. Instead, a
relatively flat pattern is observed, slightly enriched in accordance with summer
precipitation, and at greater depths tending to the groundwater average.
The influence of isotopic fractionation, selective root uptake and/or flow
mechanism of rainfall was evaluated by comparing the weighted average S180 values of
precipitation and soil water. Mean 8^0 was calculated for all samples, for the
centrifuged samples, for the vacuum distilled samples, and for all profiles separately
(Table 2). Comparison of precipitation and soil water values resulted in -7.3%o for
precipitation over 1992-1994 (Table 1) and -7.1% 0 for all soil water samples together.
The vacuum distilled samples were not depleted relative to rainfall (-6.9%o).
The overall averages were also calculated for the forested and non-forested
profiles to evaluate possible differences related to interception fractionation. The
average of the forested sites (-7.04%>) is slightly heavier (0.2%o) than the average of
./. C. Gehrels et al.
588
the non-forested sites (-7.22%o). This small difference is in close agreement with the
findings of Saxena (1987) and DeWalle & Swistock (1994). Apart from the small
difference between forested and non-forested sites, it can be concluded from Table 2
that the ô180 composition of soil water is quite similar to the 5180 composition of
precipitation. This is remarkable, as the profiles were taken in summer which could
have led to more enriched values. The processes of isotopic fractionation (leading to
enrichment) and/or selective root water uptake of heavier summer rainfall (leading to
depletion) seem to be of minor significance or cancel each other out.
Table 2 Calculated 5mO values (%o) of soil water sampled in the summer of 1993.
Sampling date
S1S0
n
Sampling date
S ,8 0
-6.77
-7.31
-7.54
n
17 August 1993
-6.63
19
20 Sept. 1993
15
19 August 1993
-6.91
22
24 Sept. 1993
21
-7.07
26 August 1993
20
21 Sept. 1993
18
26 August 1993
-6.86
28
18 August 1993
-7.42
17
23 Sept. 1993
-7.36
18
23 August 1993
-7.40
20
23 Sept. 1993
-6.89
20
Centrifuged
All samples
Vacuum distilled
Forested Non-forested
,8
-7.08
-7.18
-7.04
-6.90
-7.22
S 0
164
54
218
n
117
101
The weighted average SlsO of precipitation for 1992-1994 is -7.3 %<, (see Table 1).
Woeste Hoeve
Ugchelse Bos
Radio Kootwijk Cf
Radio Kootwijk VD1
Asselsche Heide
Amersfoortse Weg
Seasonal changes in the 8lsO profile
In 1994 8180 profiles were drilled bimonthly at two locations. As profiles below
short vegetation covers are likely to provide the best information, the profiles were
taken below Molinia purple moor grassland (at Radio Kootwijk) and Calluna
heathland (at Asselsche Heide) (Fig. 3). The encountered ô180 profiles show a
marked cyclicity throughout the year. Although there are many points not following
the average seasonal cycle, especially in the top of the profile, the isotope profile
usually allows interpretation of summer and winter infiltration patterns. The cycles at
Radio Kootwijk seem more regular than at Asselsche Heide. The lower infiltration
velocity (see next section) and the less homogeneous vegetation cover of Calluna
(with a more variable lateral root distribution and perhaps also deeper roots) may
explain the less pronounced cycles found at Asselsche Heide. Differences in lithology
may also be of influence here, although the profiles at Radio Kootwijk and Asselsche
Heide are quite comparable, with the top 2-3 m consisting of well sorted aeolian
cover sands and fluviatile material below (Gehrels, in preparation).
The weighted average soil water 5 values, calculated for each profile (Table 3),
show that the 180 content of soil water fluctuates throughout the year. The winter profiles are generally more depleted than the summer profiles. An influence of recent
precipitation is expected, since the profiles contain only a few years of infiltration. At
777e mechanism of soil water movement as inferredfrom ,sO stable isotope studies
589
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J. C. Gehrels et al.
Table 3 Weighted average soil water 5I80 content determined for the six profiles (depth 8 m) drilled
at Radio Kootwijk (Molinia) and Asselsche Heide (Calluna) in 1994.
Radio Kootwijk
12/01/94
-8.64
01/03/94
-7.97
02/05/94
-8.34
04/07/94
-7.71
02/09/94
-7.28
03/11/94
-7.89
mean
-7.97
13/01/94
28/02/94
03/05/94
05/07/94
03/09/94
02/11/94
mean
-8.09
-7.85
-7.48
-7.54
-7.12
-7.71
-7.63
Asselsche Heide the variations are less than at Radio Kootwijk, suggesting that the
profiles at Asselsche Heide contain a longer infiltration record. The overall average
soil water 8lsO content over the six profiles at Asselsche Heide (-7.6%o) is virtually
the same as the (1993-4995) average values of precipitation (-7.6%o) (Table 1) and
groundwater (-7.5%o). At Radio Kootwijk the profiles are depleted (-8.0%o).
Depletion of the isotope content cannot be attributed to fractionation of any kind, as
these processes unidirectionally lead to enrichment. The encountered depletion is
therefore attributed to preferential root uptake of rainfall. The depletion suggests that,
on average, water infiltrated in summer is preferentially consumed by the vegetation. A
rough indication can be obtained of the distribution between winter and summer
infiltration from the degree of depletion. Since precipitation has ô values of, on
average, c. -8.8%o in winter and c. -6.2%o in summer, a ô value of -8.0%o at Radio
Kootwijk would then represent soil water of a winter to summer mixture of
roughly 2:1.
The process of selective root uptake must be related to the soil water infiltration
mechanism. While soil water movement through the root zone at Radio Kootwijk
may be described by predominantly piston displacement, summer water at Asselsche
Heide passes the root zone (almost) as quickly as the water infiltrating in winter. A
factor that possibly enhances the differences between Radio Kootwijk and Asselsche
Heide is that the growth of Molinia (at Radio Kootwijk) is more seasonally driven
than the growth of Calluna (at Asselsche Heide). Molinia completely dies off in the
autumn and évapotranspiration only starts in late spring.
Simulating soil water movement
The 8180 profiles were simulated with the time-depth model of equations (2) and (3)
and fitted with a Marquardt-Levenberg optimization routine. Both vs„,and b were
optimized. The trends in the simulated and observed isotope profiles show a clear
resemblance (Fig. 3). The calculated soil water velocities vv„,, the measured average
soil water contents 9 and the resulting recharge rates R are presented in Table 4.
Not only monthly precipitation records were fitted, but also moving averages of these
curves with three moving average terms. In some cases the latter procedure gave
better results. The moving average procedure can be seen as a conceptual way to
The mechanism ofsoil water movement as inferred from ,sO stable isotope studies
591
Table 4 Groundwater recharge estimated for the 1994 profiles at Radio Kootwijk and Asselsche Heide
with the time-depth model of equations (2) and (3). Recharge rates were calculated from the estimated
soil water velocity and the measured soil water content.
a
v
'™
1
(m year )
Radio Kootwijk:
January
5.58
March
7.26
May
8.11
4.62
July
4.49
September
5.85
November
Asselsche Heide:
January
3.88
3.72
March
May
5.36
4.24
July
3.17
September
4.61
November
r
i!
1
(m year )
-
0
CT
-
R
e
1
(m year" ) (m year1)
a
«
0.215
0.275
0.174
0.097
0.096
0.094
0.937
0.608
0.621
0.719
0.201
0.851
0.089
0.074
0.087
0.061
0.064
0.077
3.00E-3
2.97E-3
4.70E-3
3.96E-3
4.67E-3
9.76E-3
497
535
702
282
289
449
25
30
41
19
22
58
0.199
0.470
0.487
0.193
0.173
0.040
0.883
0.832
0.009
0.782
0.417
0.513
0.083
0.071
0.082
0.074
0.068
0.084
5.53E-3
4.91E-3
1.04E-2
7.71E-3
8.34E-3
1.12E-2
320
265
442
312
215
388
27
38
69
36
29
52
incorporate dispersion in the fitted profile.
The vvw values in Table 4 are given with the parameter estimation error a F
resulting from the least-squares fit according to equations (2) and (3). The mean soil
water content, 0, is determined as the average of observed 9 below the root zone
and cjg is the standard error in the mean, representing the uncertainty in
estimating 9. R is calculated as vyl),-9; aTt is calculated fromCT,7and a-e as:
2
7\7
2
—2
2
2
2
a|=9--0-F+v;i,.ae-+a,7-a§
The parameter aTl thus represents the uncertainty in estimating R from both
simulating vV)), and measuring 9. The cross correlation between vvll, and 8 was
neglected for reasons of simplicity.
In some cases, the correlation coefficients are not very high (Table 4), which is
partly related to the use of monthly rainfall data. The use of monthly values will lead to
deviations especially in the top of the profile where individual rainstorms may
determine the isotope content. Another cause of deviations is the use of the Groningen
precipitation record instead of a record from rainfall at The Veluwe. However, since
the longer term seasonal fluctuations are of interest for this study, these deviations are
not likely to significantly influence the results. Two of the r2 factors are very low
(Radio Kootwijk in September and Asselsche Heide in May), for which the reason
must be that these two summer profiles show very limited cyclicity. It is likely that the
profiles are much more distorted during summer when infiltration fluxes are low and
evaporative demands are high. When cyclicity disappears, we actually fit a model to a
noisy straight line, giving zero correlation. Despite the sometimes low correlation, the
trends are mostly unmistakably similar. The curve-fitting process has the usual
592
J. C. Gehrels et al.
drawbacks though, such as the question of whether or not a "single solution" is
obtained. Sometimes a local minimum was indeed found, but by repeatedly starting
from different initial values, satisfactory results could be found.
DISCUSSION AND CONCLUSIONS
For areas with cool to temperate climates the isotope concentrations of rainfall are
seasonally "tagged" because of the temperature dependence of isotope concentrations
of precipitation (Dansgaard, 1964). Therefore recharge waters from successive
seasons can be identified in deep unsaturated profiles, notably where piston flow is
the dominant mechanism of water movement. However, diffusive attenuation and
dispersion of the signal through preferential flow paths and preferential water uptake
mechanisms are to be expected.
Preferential root water uptake of summer precipitation
Does the vegetation preferably consume summer infiltration? Zimmermann et al.
(1967) reported that the composition of groundwater in Central Europe typically
shows the average rain 2H composition: "This is somewhat surprising, since one sees
from the tracer work on soil-water movement that parts of the annual precipitation,
e. g. most summer rain, do not reach the water table at all, but evaporate completely.
This tends to make the groundwater isotopically lighter than the yearly rain
average." And yet, Darling & Bath (1988) found that aquifer recharge in eastern
England was closely related to weighted annual average rainfall and had remained
constant for many years, from which they concluded that rainfall contributes to the
isotopic composition of recharge during the whole year.
The recent measurements at The Veluwe confirm these findings. Groundwater
sampled previously and in the present study produced an average composition of
c. -7.5%o, with an indication that the shallower (more recent) groundwater is slightly
lighter. This value is not significantly different from the average long-term
precipitation composition and is certainly not depleted. The groundwater isotope
content is virtually in the middle of average winter (—6%o) and summer (~ -9%o)
precipitation, suggesting that in general winter and summer precipitation equally
replenish the groundwater. However, especially at Radio Kootwijk, with a vegetation
cover of Molinia, the soil water profiles seem to be depleted down to an average value
of c. -8.0%o. The seasonal cyclicity as apparent from the isotope profiles at Radio
Kootwijk indicates that the mechanism of soil water flow is characterized by piston
displacement accompanied by a diffuse attenuation. Both the depletion of the profiles as
well as the piston flow movement suggest that, at least at Radio Kootwijk, preferential
root uptake of summer rainfall does occur.
The explanation for the deviation of the results of the Radio Kootwijk site from
the majority of those from The Veluwe is the difference in vegetation. Firstly,
below the forested areas at The Veluwe the isotope content is expected to be
The mechanism of soil water movement as inferred from lsO stable isotope studies
593
enriched by 0.2-0.3%o because of interception evaporation, as was observed in the
1993 profiles in accordance with e.g. DeWalle & Swistock (1994). This will
(partly) counterbalance a preferential summer uptake effect. Secondly, the profiles
below forested sites exhibit a more dispersive character. Piston displacement is
likely to be less important and preferential flow probably dominates. Preferential
uptake of summer rainfall, which is connected to piston flow movement, is
therefore not very likely to occur. Asselsche Heide, with a vegetation cover of
Calluna shows an intermediate position. The average 180 content equals the
composition of precipitation, but the seasonal variation can still be identified. This
suggests the occurrence of both preferential and diffuse, piston-like, flow
components.
Flow mechanism deduced from the isotope profiles
The non-existence of a significant difference between the isotopic content of precipitation and groundwater suggests that (at least part of) the infiltrating summer
rainwater passes the root zone quickly and contributes to the groundwater recharge.
This conclusion is supported by a large number of chloride profiles that were also
established in the framework of the present project (Gehrels, in preparation). The
chloride distribution values show high concentrations in the root zone, but these drop
sharply to lower values below the roots to rather constant concentrations. Similar low
CI" concentrations are found in the saturated zone. The decreasing values can only be
explained by preferential flow of less concentrated rainwater bypassing the root zone
via preferred paths, avoiding water repellent soil aggregates in the podzolic humic
horizons of the root zone. The plant roots are most likely surrounded by slowly
migrating and evaporating water from both winter and summer origin, producing
high Cf concentrations.
At the non-forested sites the seasonal 8180 cycle is clearly identifiable below the
root zone. In a situation where preferential flow dominates, the seasonal cyclicity
would be more likely to be absent. Hence it is concluded that below the root zone,
where more homogeneous conditions prevail, the preferential flow component
disperses to a more diffuse (piston) type. This interpretation is very much in accordance with the findings of Ritsema et al. (1993), who found a similar pattern after
application of a bromide tracer into a water repellent sandy soil. At the locations
with forest vegetation, the seasonal cycles are very distorted. The roots are active to
a much greater depth and the lateral distribution of the roots is much more variable.
Preferential flow is expected to dominate to a greater depth and, because of the more
irregular lateral distribution, piston displacement may be absent in the percolation
zone as well.
Summarizing, preferential flow dominates in the root zone below low homogeneous vegetation covers. Below the root zone, the soil water movement is
dominated by diffuse (piston-type) flow. Soil water flow is more dispersive below
forest covers. Here the stable isotope fluctuations quickly disappear due to the dispersive effects of the deep root system.
594
J. C. Gehrels et al.
Acknowledgements This study was carried out in the framework of a PhD study on
the behaviour of groundwater level fluctuations in the central part of The Netherlands
(Gehrels, in preparation). The authors are indebted to Eric van Zanten and Adriaan
Knibbe who analysed some of the isotope data. We would like to thank the staff of
the CIO in Groningen for their helpful cooperation. The paper benefited from the
helpful comments of Frans van Geer.
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